Conventionally, color printing on a color printer is performed using print data generated on a personal computer, for example. The print data is formed of either color image data (input image) generated on the personal computer, or color image data inputted into the personal computer from an external source. The color image data is formed from RGB values, which are the three base colors red, green, and blue. Since the base color configuration for rendering colors is different depending on whether the colors are rendered with an optical system or a printing system, print data cannot be formed of RGB values. Accordingly, print data is created by converting the RGB values to a color system used in color printers, such as CMY values having the base colors cyan, magenta, and yellow or CMYK values having the base colors cyan, magenta, yellow, and black. Generally, RGB values are converted to CMY values by referencing a look-up table storing correspondence relationships between these values.
The look-up table described above functions to convert R, G, and B values from the RGB color space to C, M, and Y values constituting the CMY color space. However, since a look-up table for converting 0-255 8-bit signals of the R value, G value, and B value to 0-255 8-bit C, M, and Y values would be considerably large, computers ordinarily use a look-up table storing CMY values corresponding to only representative points in the RGB values. CMY values corresponding, to RGB values other than the representative points are calculated through interpolation.
Since CMY values computed through interpolation are displayed in a finite number of digits (8 bits), digits in the computed numbers that are below the significant digits are either rounded up or rounded down, resulting in rounding errors. In other words, the calculated results often deviated from the correct values. As a result of this deviation, the color of the actual printed image (hue and tone) is different from the color of the inputted image, thereby reducing reproducibility of the inputted image. The drop in tone reproducibility in particular results in a noticeable drop in image quality. Therefore, when performing a conversion process using a look-up table to convert RGB values to CMY values, the generated CMY values are sometimes expressed with a greater number of bits (an increased number of effective digits), such as 10 or 12 bits, as described in U.S. Pat. No. 5,075,767 (corresponding to Japanese Patent Application Publication No. 63-304773). This method can reduce error between the calculated result and the correct value and improve color (tone) reproducibility in the printed image, thereby forming printed images of a higher quality.